1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, to a method for manufacturing a NAND type semiconductor memory device.
2. Description of the Related Art
NAND type memory devices such as read-only memories (ROM's) and flash memories have been broadly used, since NAND type memory devices have higher integration than NOR type memory devices.
In a prior art NAND type ROM device, at least one series of memory cells are connected to a bit line, and therefore, a memory cell current flowing through the memory cells, i.e., a current flowing through the bit line is sensed. However, when the integration is advanced, such a memory cell current is remarkably reduced, for example, to about 10 to 20 .mu.A. As a result a high precision sense circuit is required to sense such a small memory cell current; however, this type of high precision sense circuit is subjected to noise, thus inviting erroneous operation. This will be explained later in detail.
On the other hand, in order to enhance a sense current flowing in an N-channel MOS transistor, a BiMOS device is known where a base of a PNP-type bipolar transistor is connected to the drain of a MOS transistor (see: N. Rovedo et al., "PROCESS DESIGN FOR MERGED COMPLEMETARY BICMOS", IEDM Technical Digest, 1990, pp. 485-488). Therefore, if this CMOS configuration is applied to a NAND type ROM, a sense current flowing through the bit line is substantially increased.
When manufacturing the above-mentioned NAND type ROM device where a bipolar transistor is connected between the bit line and the memory cells, after a shallow base region of the bipolar transistor is formed, an emitter electrode is formed on the base region. Then, a deep drain region of the MOS transistor is formed and is connected to the base region of the bipolar transistor (see: FIG. 3 of the above-mentioned document). As a result, if the drain of the MOS transistor is deviated from the base region of the bipolar transistor, the operation becomes unstable.